Michael Bradley will be the first US international (Not including Giuseppe Rossi, who left for Italy and plays for Italy) to start a game in Serie A since Alexi Lalas in 1994.

Onyewu played in a champions league game and the World Football Challenge but never started a game for AC Milan in Serie A.
I'm thinking he will be a solid midfielder for them, as long as he gets playing time, this can only be good for US soccer players looking to play in Europe.